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Old 09-26-2012, 07:21 AM   #16
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Check PS to see if it is making circuit during fan 20 second run time. OR JUMP PS just after ID fan starts running. You can't jump it before hand as this will be detected by "board" and it will not allow continuation of sequence/ignition.

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Old 09-26-2012, 08:15 AM   #17
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Thanks these are all good suggestions. My instinct is that this is the ID motor that is bad. I considered jumping the PS during the 20 sec run and seeing if ignitor works. I was afraid to jumper wires when I don't fully understand the schematic. Fear of destroying the circuit board. Now I understand this more clearly. The PS is a relay that connects voltage to the ignitor. The PS is definitely not closing during operation as I measured 26V across the terminals during operation. If it was closed, there would be no voltage across the switch terminals. The PS is functional. I don't have a manometer but I did the mouth test. It didn't take much suction to close it. I also removed red hose from ID and ran motor and with ID running I slowly moved my index finger over hole to see if I could feel any suction from ID (I didn't). I was thinking I would feel something. Maybe I should have put a sheet of paper in front to see if it is drawn in. I don't have a full suite of HVAC tools. I am a DIY person

Next steps:

1. Bypass ID by either a) jumping terminals on switch during 20 sec run
b)using mouth to pull switch in during 20 sec run

(I am reasonably sure switch will work)

2. If switch works and furnace ignites (I am pretty sure it will)
a). check for blockages again in flue. Remove cap on roof and look down in. I previously removed flue on furnace end and looked inside. There was minimal debris and no blockage.

b) Focus on ID motor. Check for debris and blockage at the motor. I am not sure I can jump an extension chord directly to it. This will take some thought how to accomplish.

Thanks for all the suggestions. I have to work at my regular job now. Probably won't get back to this until the weekend.
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Old 09-26-2012, 05:57 PM   #18
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


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Originally Posted by Muzzle127 View Post
I removed the pressure switch and connected an ohm meter across the switch contacts. Then with my mouth pulled air thru the hose and heard the switch click and the resistance went to ~0 ohm. I also re connected switch and measured voltage across the contacts. Voltage was ~26 volts
Unless this is some sort of unusual vacume switch it is nothing more than an On/Off micro switch. You need to measure voltage from terminal to ground, not terminal to terminal. Terminal to terminal with a voltage tester just bypasses the switch. Check each terminal individually. I think you will find that the switch is not proving when the draft inducer is running. Do you have a picture of this setup you can post.
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Old 09-26-2012, 09:19 PM   #19
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Clean the port out that the red hose hooks up to. Inducer on with no suction at the hose means the port is plugged or the hub on the inducer wheel broke. Even if the flue was blocked there would some suction there.
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Old 09-26-2012, 09:22 PM   #20
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


see post #15 marty and I think alike..... pluged port
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Old 09-26-2012, 09:27 PM   #21
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


read post #8.
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Old 09-26-2012, 09:31 PM   #22
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


ok doc 3 heads are better that 1-or 2 ...brillant minds think alike:laughing : even if its slow thinking
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Old 09-26-2012, 09:43 PM   #23
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


are we the 3 stooges or the 3 musketeers?
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Old 09-26-2012, 10:07 PM   #24
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


You guys are comical. The sucking on hose and breathing thru nose is called circular breathing. Sax players do that trick all the time. I have a piece of copper wire 14 gage that i jabbed into the ID port. Maybe I wasnt thorough enough. I will try again next time I go into my attic. Until then I will use my auxilary heater.

Last edited by Muzzle127; 09-26-2012 at 10:09 PM.
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Old 09-26-2012, 11:26 PM   #25
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


remove the stat off the subbase and jump R to W the ID should be the first thing to run and not stop till the stat is satisfied from a heat cycle
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Old 09-27-2012, 12:52 PM   #26
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Got up in attic at lunch. Bypassed pressure switch w alligator clip and the ignitor heated and burner fired. So the ID motor is suspect again.

I checked the port on ID and put a paper clip in there. No blockages found. I also again pulled the B vent off the furnace side just to see if theres a blockage on outlet side in the pipe going to roof. The pressure switch still wouldnt close. Next I will remove the ID motor from the furnace and inspect. Just 3 nuts to take off.
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Old 09-27-2012, 12:56 PM   #27
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Jumpered PS and ID mot
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Last edited by Muzzle127; 09-27-2012 at 12:58 PM.
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Old 09-27-2012, 02:03 PM   #28
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


I think you are on the right track..you may find several fan blades broken/distorted...maybe a bird/squirrel/RAT? ..lol...BE PREPARED!

All the best! Techpappy
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Old 09-27-2012, 09:20 PM   #29
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Removed Id motor and found the felt gasket between furnace and motor in pieces. I think a piece got sucked over exhaust hole. This would explain no draw or exhaust. Possibly the air coming from the side of the fan too next steps are to test motor with straight 120 v from extension chord. Tomorow i will buy a new felt gasket.
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Old 09-27-2012, 09:25 PM   #30
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too


Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too-image-1804826971.jpg

Back of id and gasket pieces
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Lennox G50 wont ignite. Pressure switch is ok, intake clear, exhaust clear too-image-926246709.jpg  

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